Direct access storage devices (DASDs), such as hard disk drives, typically fall into two classifications. The first classification is count key data (CKD) direct access devices. The second classification is fixed block architecture (FBA) direct access storage devices.
CKD direct access storage devices can be differentiated from FBA devices by the manner in which data is accessed from the device. In a CKD device, individual records or information are accessed through the use of an address which uniquely identifies the physical location of the record within the device.
The information is accessed from an FBA device by specifying a relative block number. The FBA device itself then converts that relative block number into the actual physical location of the requested data. When an FBA device is manufactured, the storage media is physically divided into fixed memory locations.
In the past multiple users typically worked on a single computer system under the control of a single operating system. It was easy for users to share data in this type of environment, because the operating system could control all the users' access to the data and insure that they all had appropriate access while maintaining data integrity.
As computer system costs have decreased, it has become advantageous to give each user his own system. Users still need to share data, but a single operating system is no longer in control. What is needed is a way for the separate operating systems to insure that users do not acquire conflicting access to the shared data and open the door to possible data corruption.
This need has also existed to some extent for very large (main frame) systems. However such systems typically use count key data (CKD) direct access storage devices, have trained operators to monitor the system, and only a few systems (typically up to 16) can share data at one time.
Therefor what is needed is a system that can support Fixed Block Architecture (FBA) DASD, does not require monitoring by trained operators, and which can accommodate more than 16 systems. These requirements encompass the needs of a typical workstation LAN (local area network) environment such as the Personal/370 (P/370) processor (available from IBM Corporation of Armonk, N.Y.) operating in a LAN.